The present invention relates to circuits and methods for providing hi-directional drive currents to an inductive write head.
Magnetic disk drives have been steadily improved since the 1950s, but most of the basic principles of operation have not changed. A head which includes a solenoidal coil is located in proximity to a rotating platter coated with a ferromagnetic or ferrimagnetic medium. By driving current through the coil, a magnetic field can be generated at the surface of the magnetic medium which is strong enough to induce a transition in the magnetization of the medium. By reversing the current in the coil, the direction of magnetization of the medium can be changed. The domain boundaries thus created can be sensed for reading, and are reasonably stable, and thus provide nonvolatile storage of data. Write amplifiers for magnetic disk drive heads must therefore drive rapid transitions of the current sense across the inductive write head.
In a disk drive, traditionally the head was a coil (or more recently a thin film head which is equivalent to a coil), embedded in some form of a head that slid across the top of the disk platter, and positioned to create a magnetic field in a small area of the surface of the platter. By controlling the amount of current that flows to the coil, and switching it from one direction to the other direction, a series of magnetic dipoles would be created in the ferrimagnetic medium at the surface of the disk.
Normally a "1" is indicated, on the disk, by a transition in the magnetic field. No transition would imply a zero. (These transitions are synchronized in ways not relevant here.)
A disk drive normally includes multiple head elements each mounted on respective arms. The arms move across the disk and trace out various rings of magnetic data. If we could see the magnetic domain boundaries in the magnetic medium on the disk, we would see chains of overlapping circles, almost like overlapping punch-outs, where the write head changed its magnetic field and pushed out a new flux domain. The written domains are spaced closely enough to overlap (and therefore very few of them are circular), but there is enough remaining area in each one to preserve the written data.
As manufacturers move to higher disk rotation RPMs and/or smaller physical dimensions to which the write head is magnetically coupled, the bandwidth required for the write head steadily increases. The demands on the write amplifier are fairly severe, and are rapidly becoming more so with improved head and magnetic film technologies. The present invention provides a high speed design, which can obtain very high bandwidth from conventional silicon technology, and provide transitions of about 100 million bits per second with typical head inductances, and 5 potentially up to several hundred MHz for small inductances.
FIG. 1A shows a simplified circuit diagram of the connection of a write amplifier to a thin film disk head. The head appears to the amplifier as a predominantly inductive load of relatively large value (e.g. 600-800 nH).
FIG. 1B shows a more detailed model of the electrical properties of the thin film disk head, including its connections. Typical magnitudes for the reactance components shown are:
C.sub.(leads) .apprxeq.5 pF; PA1 .sub.L(leads) .apprxeq.700 nH; PA1 R.sub.(head) .apprxeq.40.OMEGA.; PA1 L.sub.(head) .apprxeq.100 nH each.
As of 1995, the read and write data rates are typically in the neighborhood of 180 megabits per second. Since each edge (rising or falling) corresponds to one bit, the frequency is therefore typically in the neighborhood of 50 MHz. Achieving this speed with the significant inductance of the head coil is not easy.
Inductive voltage is often stated as V=LdI/dt; but to properly analyze the rise time effects in an inductive load, the basic inductance relationship should be stated in a way which preserves causality: ##EQU1## That is, the correct causality (in a write head) is that the voltage is changed, and this produces a change in current. Thus when the inputs to an inductive load are switched, the voltage can be changed quickly, and the current will then ramp up in proportion to the time integral of the applied voltage.
FIG. 2 is a current timing diagram, showing how the head current behaves during two transitions in opposite directions. Once the applied voltage reaches its maximum value, the current changes at a steady rate dI/dt.
FIG. 3 shows an ideal voltage waveform for write drive. Suppose that the coil current has been constant for a relatively long time: the inputs are in one direction, and the voltage across the inductor is just an IR drop. When we switch the inputs, initially the current can't change in the inductor, so the inductor looks like an infinite resistance and the driver will rapidly slew to its full voltage swing V.sub.sat. This voltage V.sub.sat causes a dI/dt in the inductor, and the current will change at a steady rate until the amplifier approaches its current limit (a value which is programmed by the external resistor, in the presently preferred embodiment). Now as the current begins to level off at the new current level, the voltage across the inductor drops, until its connections reach the same IR voltage magnitude which was originally present. (However, this new IR voltage has the opposite sign to that previously present.)
The magnitude of the amplifier's peak voltage V.sub.sat will determine the rate at which we can change the coil current. Typical values for T.sub.SLEW in 1994 are in the range of 4 to 9 nsec.
The voltage slew rate of the write amplifier is also significant, since this too is a component of delay and limits the data rate. However, the time at V.sub.sat, during which the current is changing at its peak rate, will normally dominate the switching time.
Thus one important objective is to provide a sufficiently high magnitude of Vsat. A secondary objective is to provide an adequately high voltage slew rate at the amplifier output. A further objective is to provide an accurately regulated peak current. The disclosed circuit embodiments provide innovative circuit ideas which are advantageous in meeting all of these objectives.
Propagation delay in the path into the write amplifier is relatively unimportant, since it does not affect the data rate.
To maximize switching speed in bipolar technology, it is desirable to keep the bipolar transistors from saturating. Moreover, the use of bipolar rather than MOS transistors provides additional headroom, since the diode drop of a bipolar transistor is only about 0.7 volts, as compared to a MOS threshold voltage of typically 1.0 or 1.1 volts. The present invention drives the pull up transistors directly with a differential signal obtained by standard techniques, but uses a shifted and scaled version of the base voltage of the pull up transistors to drive the pull down transistors. Moreover, a current source regulates the current which can be drawn by the pull down transistors.
It is also preferable to use NPN rather than PNP devices in speed-critical paths. Some attempts have been made to use PNP devices in bipolar drivers, but the problem when high frequencies are required is that PNP drivers necessarily exhibit longer switching times, due to the lower mobility of holes. Thus, while the size of PNP drivers can be adjusted to provide a gain which is reasonably balanced with that of NPN drivers, mere sizing adjustments cannot solve the problem of switching time.
In magnetic write applications, it is desirable to carefully control the maximum current provided. However, in other applications, this may not be necessary.
Innovative Write Amplifier
The present application discloses an innovative circuit for driving the write head. In this circuit, all of the driving transistors are NPN, and all are prevented from saturation. This maximizes their switching speed. This is achieved by shifting and scaling down the differential drive applied to the pull-up transistors, to drive the pull-down transistors with levels such that the pull-down transistors cannot reach saturation. This provides a very simple circuit in which all four of the drive transistors are NPN, and all are kept out of saturation. Moreover, the peak write current applied to the head is precisely limited.
Note that the innovative circuit is not a switching circuit, and not an H-bridge, but rather a fully differential double-ended driving circuit. By contrast, an H-bridge has four inputs and they are driven by separate circuits which must be synchronized. By putting the level shifter right at the immediate output stage and decoding a differential signal to drive it, we have bypassed all of that. H-bridge switching circuits are conventionally used for low-speed bidirectional switching; but for high-frequency capabilities such as is required by write amplifiers, such switching circuits are wholly inadequate.
Since the properties of magnetic media will vary between manufacturers, and as processes and materials are optimized, the write amplifier, in the presently preferred embodiment, provides precise mirroring of current drawn on an external pin, to accurately define the current that would flow in the coils. Thus, the drive manufacturer can accurately control the write current by changing the value of an external precision resistor to the ground, or alternatively by connecting a programmable current sink to this pin. Thus the drive manufacture can change the current for different head medium characteristics.
The necessity for reversal of current direction on a differential output appears in other applications also, including drivers for voice coils and other small electromechanical actuators, ultrasonic transducers for imaging at VHF and higher frequencies, and switching or phase-shifting of microwave signals. The disclosed circuit can be advantageous in these and many other analogous applications.